AstraMike:
The mentioned Russian aircraft that crashed following a “flat spin” entered at altitude, in turbulence, had a Second Officer who was a qualified aerobatics pilot - not that it helped.
I thought I read that the 2nd Officer was a former national champion at aerobatics, which strengthens your point about how hard it is to recover. I cannot personally imagine any passenger liner recovering from a fully developed flat spin (maybe a fighter with thrust vectoring might be able to do it, I don't know).